Cooling load and selected fan coils

[rami_9630]

Hi guys, i have very well insulated residential building where district cooling system and fan coil units are used for air conditioning. Due to high performance of the heat insulation used, the cooling load calculated were very low. when i came to selecting the fan coil unit in each room, i had to choose ones of capacities that are higher than the cooling load of the room by around 50%. The heat exchangers and water pumps are sized based on the calculated cooling load. Now, the problem is that the chilled water flow of the system is 50% less than the chilled water flow required by sum of all fan coil units. i know fan coil units wouldn't work all together but I'm still concerned about some issues that may arise due to this big difference between calculated loads and selected fan coil unit capacities. Please note that I'm using variable speed pump with two way valve for each fan coil.
is there any problem with such system, your advise is highly appreciated
Thanks,

[dan sw fl]

See diversity on page 8.
http://www.mcquay.ru/downloads/WSC%20design.pdf

[rami_9630]

See diversity on page 8.
http://www.mcquay.ru/downloads/WSC%20design.pdf

Thanks dan for your reply, i did detailed calculation and the envelope is defined, i had to go for a fan coil unit which is double the cooling load of the room because it was the smallest one available and i don't want two rooms to share same fan coil unit. the building is huge one of 30 stories and i have hundreds of fan coil units. i have also another concern about the fresh air handling unit, since the system chilled water flow is less than whats required by total fan coil units, this may create a shortage in water flow required by fresh air handling unit which would affect the ability of the coil to remove moisture.

[dan sw fl]

Thanks dan for your reply, i did detailed calculation and the envelope is defined, i had to go for a fan coil unit which is double the cooling load of the room because it was the smallest one available and i don't want two rooms to share same fan coil unit. the building is huge one of 30 stories and i have hundreds of fan coil units.

So you are an Expert on Chilled Water System Design and there won't be an issue since the design will have MULTIPLE reviews and challenges by other experienced professional engineers and inspectors prior to sizing the chiller, pumps, piping, controls, set-up and commissioning. Sounds like VRV systems would be appropriate.

I cannot even imagine how small the rooms are going to be, must be < ~200 square feet.

[motoguy128]

I'm agreeing. Rather than a fan coil, it seems like VRV unit utilizing 1 air handler for each space with hot water reheat might be appropriate maybe using hot water reheat at each VRV.

[teddy bear]

Thanks dan for your reply, i did detailed calculation and the envelope is defined, i had to go for a fan coil unit which is double the cooling load of the room because it was the smallest one available and i don't want two rooms to share same fan coil unit. the building is huge one of 30 stories and i have hundreds of fan coil units. i have also another concern about the fresh air handling unit, since the system chilled water flow is less than whats required by total fan coil units, this may create a shortage in water flow required by fresh air handling unit which would affect the ability of the coil to remove moisture.

Currently involved in a retirement center with a similar problem. They are supplementing their make-up air system with several large dehumidifiers. Their chilled water in not cold enough and the cooling load not high enough to get down to <50%RH. They had +65%RH commonly in the isolated rooms when the outdoor dew points were +55^F. The removal of moisture is dependent on the chilled water temp leaving the cooling coil and the length of the coiling cycle.
The fix is to thoroughly dry the incomming fresh air (<45^F dew point) and provide extra dehumidification to all the open parts of the space. Reheat without a very cold chill water will provide <50%RH. Reheating only make the chilled water system warmer which removes less moisture. This is critical in green grass climates.
Regards TB

[rami_9630]

the room not very small but the load is, the building can fit for leed rating due to superior characteristics of the envelope, the glass u-value is 1.8 w/m2k, SC=25 and wall u-value is 0.3 w/m2k, so you can imagine how small loads can be.

[rami_9630]

thank you all for replying, im thinking of adding secondary pumps of flow capacities higher than the primary pumps to deal with flow demands of fan coil units. i am not sure if secondary pumps flow can be greater than primary flow, any ideas?

[motoguy128]

Is there a storage tank seperating the primary and secondary? If so, I dont see a problem. In com/ind with multiple chillers you size primary pumps for each chiller then secondary pumps for the cw loop. But you normally have a storavge tank to provide some resonance to reduce short cycling. At
Least thats how I understand it.

It could be 2 or 3x the flow for whst it matters. Buy the delta of course would be smaller since capacity is seyt by the chiller.



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